Like a hand-held mower (shears) and a knapsack operation machine targeted to vert, conventional two-stroke and four-stroke small-size general engines have been widely used as engines of the operation machines that drive operating personnel to hold by hands or bear on the back for operation. In order to ensure the normal operation of the engine, it needs to be lubricated by lubricating oil. The four-stroke engine is usually equipped with an oil pan for storing the lubricating oil. Because the engine may be possibly used in different states, for example, used in an inverted or leant manner, the lubricating oil in the oil pan may be possibly accumulated in any position of the oil pan. Therefore, the structure of the oil pan plays a very important role on whether the lubricating oil in the oil pan can be sufficiently used.
In the prior art, arc-shaped bulges will be formed in the oil pans of some engines due to the factors of structure and processing. When the engine is used in an upright manner, the arc-shaped bulges will not affect the flowing of the lubricating oil. However, when the engine is used in a leant or inverted manner, the arc-shaped bulges will affect the flowing of the lubricating oil in the oil pan, thus causing insufficient utilization of the lubricating oil.